Digital image acquisition devices, e.g., scanners, digital copiers, and 5 digital cameras, are used to digitize the graphic content, be it color or black and white photographs, artwork, text, and other graphics, from reflective and/or transmissive original documents. This capability is useful in digital document storage, digital content generation, and in more industrial pre-process environments. In this latter implementation, chemical film-based photographs, for example, can be digitally scanned for pre-print review, followed by production printing.
Considering the example of the scanners, one of the most common configurations is the flat-bed scanner. A transmissive or reflective original document is placed face down on a bed having a glass plate. A carriage, under the glass plate, with a slit aperture facing the document, is then scanned over the original document. An optical system in the carriage successively picks-off scan lines. In the one configuration, a single fold mirror is used in a high-resolution mode, and a series of larger fold mirrors are used in a low-resolution mode. In either case, the fold mirror(s) relay the scanned lines to high or low resolution imaging lens, also of the optical system, which image the scan lines onto a linear or two dimensional image sensor. In the most common implementation, the image sensor is a trilinear charge-coupled device (CCD) array, although newer CMOS-based image sensors are becoming increasingly popular.
An exemplary flat-bed scanning system is disclosed in U.S. Pat. No. 5,696,609 to Cresens et al. and assigned to AGFA Division, Bayer Corporation. This patent is incorporated herein in its entirety by this reference.
Calibration is a substantial cost factor in the manufacture of scanners and other image acquisition devices. In a scanner, the carriage""s optical system, including the slit aperture, relay optics, and imaging optics, must be aligned to the image sensor array so that an image of the scan line is properly formed on the sensor array. Moreover, the optical system calibration must be robust. After manufacture, these devices are many times transferred by commercial shippers around the world where exposure to shock and temperature extremes take place.
The most common calibration technique in a scanner involves attaching a prealigned CCD sensor array to the carriage. The relay optics and imaging optics are then finely tuned to the required level of alignment with the attached sensor array.
A number of problems exist or arise in the course of conventional calibration of an image acquisition device, the most significant of which is the complexity of the calibration protocols. For instance in a scanner, the properly-located scan line must be picked off through the slit aperture by the relay optics and transferred to the image sensor array. Moreover, the image of the scan line must be formed such that it is in focus across the entire two-dimensional sensor array. If the image plane is decentered, clocked, tipped, or tilted relative to the plane of the sensor, re-alignment of the relay optics is performed, which can affect the location of the scan line. Thus, calibration requires simultaneously managing multiple independent variables. Specifically, for two dimensional sensor arrays, tilting is very important for color registration.
Moreover, in existing scanners, in-field re-calibration and/or image sensor array replacement/upgrading is very difficult. It is not uncommon for the sensor arrays to fail in the field. The cause can be related to shipment, normal operation, or electrical surge damage. Additionally, the image sensors are sometimes replaced to increase the scanner""s resolution as higher density sensors In these situations, the optics in conventional systems must be re-calibrated since the new image sensor array cannot be attached to the carriage in exactly the same orientation as the old sensor. Without proper calibration, the increase in resolution achievable by a new image sensor, for example, may not be realized because of poor image formation on the device. Calibration in the field, however, is even more difficult due to the unavailability of the special-purpose equipment and jigs used to facilitate calibration in the production environment.
The principles of the present invention are directed towards any image acquisition device, such as a scanner, digital copier, or digital camera. As such, the device comprises an image sensor array that detects light imaged by an optical system and an image sensor module that enables calibration of the position of the image sensor array relative to the optical system. In this way, with the optical system being constructed within tolerances, the module enables alignment of the image sensor array to the optical system. Preferably, the sensor module is separately calibrated to defined standards, making the optical system""s calibration independent of the specific module and the module""s calibration independent of the specific optical system used in a given image acquisition device.
Preferably, the module enables positioning of the sensor array with six degrees of freedom, allowing it to be aligned completely independently of the optical system. Further, the electronic circuit board, on which the sensor array is integrated, is separate from the mother board, on which the analog to digital converters are located. This has the advantage of enabling replacement of the analog to digital converters, a primary source of improper operation in the electronic components, without requiring the replacement of the expensive optical sensor array, coupled with the difficulty of re-alignment.
In general, according to one aspect, the invention features an image sensor module for an image acquisition device. The module comprises a base plate that is adapted to be mounted to an optical system, and specifically the carriage. An image sensor mounting bracket is adapted to carry the image sensor array. The mounting bracket is positionable relative to the base plate to enable alignment of the image sensor array to the optical system.
In a preferred embodiment, the mounting bracket is adjustable relative to the base plate in three degrees of freedom. Specifically, it is translationally adjustable along a z-axis, which is defined as being orthogonal to the image plane of the image sensor array; rotationally adjustable relative to a y-axis, which is defined as running parallel to a longitudinal axis of the image sensor array; and rotationally adjustable relative to an x-axis, which is defined as running parallel to a transverse axis of the image sensor array.
More specifically, a bracket position alignment system is provided that comprises at least one support pin for preloading the mounting bracket relative to the base plate and at least one set screw for controlling a distance between the mounting bracket and a base plate.
In the preferred embodiment, two support pins are used for preloading, and three set screws are used for distance control between the bracket and base plate.
Further, a base plate position alignment system enables the base plate, and thus the image sensor array, to be oriented relative to the optical system. Preferably, the base plate alignment system provides the three additional degrees of freedom. Specifically, the base plate is translationally adjustable relative to the optical system along the y-axis, rotationally adjustable relative to the z-axis, and translationally adjustable relative to the x-axis.
Thus, the base plate position alignment system in combination with the bracket position alignment system provide a total of six discrete degrees of freedom in the positioning of the sensor array relative to the optical system.
In other aspects of the preferred embodiments, the sensor array is bonded to the image sensor mounting bracket. A daughterboard, supporting the analog signal processing of the signals from the sensor array and sampling control for the array, is provided on a daughter card that is supported by the image sensor array. Preferably, the analog to digital converters that digitize these signals from the sensor array are located on a separate mother board, connected to the daughterboard by a shielded jumper. This enables the replacement of the mother board with its analog to digital converters without necessitating the removal or replacement of the daughterboard, which must be aligned to the optical system and contains the expensive sensor array.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.